Breathing porous liquids based on responsive metal-organic framework particles

Responsive metal-organic frameworks (MOFs) that display sigmoidal gas sorption isotherms triggered by discrete gas pressure-induced structural transformations are highly promising materials for energy related applications. However, their lack of transportability via continuous flow hinders their application in systems and designs that rely on liquid agents. We herein present examples of responsive liquid systems which exhibit a breathing behaviour and show step-shaped gas sorption isotherms, akin to the distinct oxygen saturation curve of haemoglobin in blood. Dispersions of flexible MOF nanocrystals in a size-excluded silicone oil form stable porous liquids exhibiting gated uptake for CO2, propane and propylene, as characterized by sigmoidal gas sorption isotherms with distinct transition steps. In situ X-ray diffraction studies show that the sigmoidal gas sorption curve is caused by a narrow pore to large pore phase transformation of the flexible MOF nanocrystals, which respond to gas pressure despite being dispersed in silicone oil. Given the established flexible nature and tunability of a range of MOFs, these results herald the advent of breathing porous liquids whose sorption properties can be tuned rationally for a variety of technological applications.


Supplementary Methods-Gas sorption measurements
In order to perform gas uptake measurements of the PLs under continuous stirring, the porosimeter, originally manufactured for the analysis of porous solids, was modified. A stirring plate was placed on the lift drive platform in place of the Dewar, and a jacketed glass flask was mounted on it and connected to a circulator to control the temperature. A float level sensor was used to control the height of the glass flask. 0.5 mL sample was injected into a 9 mm sample cell with large bulb and a PTFE stirring bar was added. The liquids (PLs and pure Silicone 704) were kept stirring during all measurements (except for the one dedicated CO2 sorption experiment of PL7_10 without stirring). Figure 1: Quantachrome Autosorb iQ MP instrument and the setup used for gas uptake measurements of PLs.

Supplementary
In the measurement routine of the Quantachrome Autosorb iQ MP a sample is considered to have reached equilibrium when the pressure change (∆p) is less than 0.81 mbar for a selected time interval (∆t) termed as equilibration time in the instrument software. Thus, the equilibration criterion is met when the magnitude of the pressure gradient is less than ∆p/∆t = 0.81 mbar/∆t). The equilibrium settings were the following: ∆t = 12 min, pressure tolerance value of 1, i.

Supplementary Figures-Powder X-ray diffraction
Supplementary Figure 3: PXRD patterns (λ = 1.5418 Å) with profile fits (Pawley method) of as synthesized and activated ZIF-7 nanocrystals. Crystallographic parameters of reference crystal structures from the literature were used as starting parameters for the profile fits (CCDC code VELVIS 1 for the as synthesized lp phase and CCDC code RIPNOV01 2 for the activated np phase). Figure 4: PXRD patterns (λ = 1.5418 Å) with profile fits (Pawley method) of as synthesized and activated ZIF-9 nanocrystals. Crystallographic parameters of reference crystal structures from the literature were used as starting parameters for the profile fits (CCDC code VEJZEQ 1 for the as synthesized lp phase and CCDC code RIPNOV01 2 for the activated np phase). The respective measurement at 25 °C is also included for the sake of comparison. Clearly, the variations in the measured viscosity at 25 °C at shear rates below 10 s -1 must be ascribed to measurement errors and not to non-Newtonian behaviour. The measurement performed at -10 °C proves the Newtonian behaviour of Silicone 704.

Supplementary Figures-DLS measurements
Supplementary Figure 13: DLS measurements of PL7_10. The sample was diluted 750-fold to obtain accurate DLS data. The calculated average particle size is 469 ± 74 nm with a polydispersity index of 0.058. Figure 15: 1 H NMR spectrum of a digested sample of ii-2-Mg2(dobpdc). Residual proton signals from DMSO and D2O are marked with ~ and °, respectively. Diamine loading is an average of the ratios calculated using each diamine signal and equals to 94 mol%, i.e. a chemical composition of (N,N-diisopropylethylenediamine)1.88Mg2(dobpdc).

Supplementary Figures-Stability of PLs
Supplementary Figure 16: Photographs of the formulated ZIF-based PLs after 3 months of storage without any agitation. The samples rested for 3 months in an isolated area in the lab. Sedimentation of the ZIF particle agglomerates is not observable.

Supplementary Notes-Gas sorption measurements
An equilibration time of 12 min was selected for our experiments. Hence, for ∆t = 12 min the pressure change should be less 0.068 mbar min -1 . The total amount of time elapsed for each pressure point was thus higher than 12 min. A column graph with the total time for each pressure point for CO2 measurement of PL7_10 at 25 °C is provided below (Supplementary Figure 22) to offer insight to the reader.
We investigated the influence of the equilibration criterion ∆t on the shape of the CO2 sorption isotherm. Specifically, we prepared A linear isotherm for CO2 sorption in silicone 704 would be expected. It appears though that the absorption of CO2 is kinetically hindered, so that the isotherm features a slight curvature and a slight hysteresis between absorption and desorption (Supplementary Figure 23) indicating that the isotherm does not represent equilibrium conditions.
In order to investigate if equilibrium can be reached by longer equilibration times, we then collected additional CO2 sorption isotherms of Silicone 704 using larger ∆t values ("equilibration times", see above) of 16 and 24 mins (instead of 12 min used previously). The results are provided in Supplementary Figure 24. It appears that longer ∆t leads to a small increase in the overall CO2 uptake at ca. 1200 mbar. The width of the hysteresis between absorption and desorption, however, is largely unaffected by the longer ∆t. This experiment established that CO2 sorption in Silicone 704 is indeed kinetically hindered, even though the liquid is continuously stirred during data collection.
Linear fits to the sorption isotherms are not satisfactory because of the slight curvature of the isotherms and the hystereses. Thus, we applied polynomial (absorption) and Langmuir (desorption) functions to derive much better fits to the experimental data.
We used the isotherm collected with a ∆t of 12 min for the comparison of the sorption behaviour of silicone 704 with the PLs reported here, since the data of the PLs have also been collected with the same ∆t setting. One might argue that the isotherms of pure silicone 704 are not fully equilibrated when the rather short ∆t of 12 min is applied, however, a reference experiment of PL7_10 with an increased ∆t (18 min instead of 12 min, see Supplementary Figures 21 and 22) shows that the increased ∆t leads to insignificant changes in the isotherm shape (i.e. phase transition pressures and hysteresis width) and overall gas uptake of the PL. Thus, all the reported gas sorption isotherms of the PLs in this work must be regarded as reasonably well equilibrated.
We may note again that breathing MOFs typically show step-shaped gas sorption with a rather strong hysteretic behaviour. The hysteresis between ad-and desorption is a consequence of the activation energy of the phase transitions (i.e. it is a kinetic effect) and real thermodynamic equilibrium is not achievable when recording sorption isotherms of such systems with experimental time scales. 4 In other words, the phase transition region of an experimental sorption isotherm of a breathing MOF is never a representation of thermodynamic equilibrium. Naturally the same is true for the breathing PLs discussed in this work. The logarithmic horizontal axis is plotted in relative pressure (p/p0). 5 The isotherm for PL7_10 recorded at 35 °C was not included because 35 °C is above the critical temperature of CO2, so that the saturation pressure p0 is not defined. For all isotherms the adsorption branch is shown with filled symbols and the desorption branch with empty symbols, while the lines are a guide to the eye. Similar to the observations for the PL7 and PL9 materials, the step during adsorption is shifted to slightly higher gas pressures, whereas the step during desorption is shifted to slightly lower gas pressures. The uptake of the PL at 1224 mbar is in good agreement with the uptake expected from the constituents, with the experimental value being equal to 93% of the ideal uptake. It is estimated that the Silicone 704 contributes a CO2 uptake of about 1.13 cm 3 (STP) g -1 PL, while the ii-2-Mg2(dobpdc) particles contribute about 3.60 cm 3 (STP) g -1 PL. This is quite impressive, since the 5 wt% ii-2-Mg2(dobpdc) particles in the PL contribute about 76% of the CO2 uptake capacity of the PL at 1224 mbar. Three adsorption/desorption cycles were carried out with the PL being regenerated under vacuum at 100 °C and 120 °C, before cycle 2 and cycle 3, respectively. Between cycles 1 and 2 the sample rested for 4 days at ambient conditions.